Electrically conductive thermoplastic compositions have previously been achieved by the addition of conductive carbon black to a polymeric base. In one category of such compositions, advantage has been taken of a non-linear positive temperature resistivity coefficient displayed by the particular material to obtain self-regulating or current-limiting semi-conductive articles. In U.S. Pat. No. 3,243,753 to Kohler, one such composition is described as containing from 25% to 75% carbon black about which the polymeric matrix has been formed by in situ polymerization. As the temperature of such a composition increases, either through a rise in ambient temperature or by reason of resistive heating occasioned by the passage of current therethrough, the polymer matrix expands at a rate greater than that of the carbon black particles which, in an interconnected array of channels, impart the property of conductivity. The resulting diminution in the number of current-carrying channels decreases the amount of power generated by I.sup.2 R heating. This self-limiting feature may be put to work in, e.g. heat tracing pipes in chemical plants for freeze protection, maintaining flow characteristics of viscous syrups, etc. In such applications, articles formed from the conductive composition ideally attain and maintain a temperature at which energy lost through heat transfer to the surroundings equal that gained from the current. If the ambient temperature then falls, increased heat transfer to the surroundings is met by increased power generation owing to the resistivity decrease associated with the article's lowered temperature. In short order, parity of heat transfer and power generation is again attained. Conversely, where ambient temperature increases heat transfer from the conductive article is reduced and the resistivity rise resulting from increased temperature diminishes or stops I.sup.2 R heating.
Self-regulating conductive compositions may, of course, be used in employments other than positive heating, for example, in heat sensing and circuit-breaking applications. In every case, however, the high carbon black content characteristic of most prior art compositions is disadvantageous. High black loadings are associated with inferior elongation and stress crack resistance, as well as low temperature brittleness. In addition, high black loading appears to adversely affect the current-regulating properties of the conductive compositions. If a semi-conductive thermoplastic composition is externally heated and its resistivity plotted against temperature (on the abscissa) the resulting curve will show resistivity rising with temperature from the low room temperature value (Ri) to a point of "peak resistance" (Rp), following which additional increase in temperature occasions a precipitous resistivity drop associated with the melt phase of the polymer matrix. To avoid resistance runaway with the concomitant irreversible change in resistivity characteristics, the practice of cross-linking the polymer matrix has grown up, in which event resistivity levels off at the peak temperature and remains constant upon further increase in ambient temperature. Cross-linked semi-conductive articles with high black loadings exhibit undesirably low resistivity when brought to peak temperature by exposure to very high or low ambient temperatures. In such instances poor heat transfer characteristics can prevent dissipation of I.sup.2 Rp generation, causing burnout.
It would accordingly be desirable to prepare semi-conductive self-regulating articles with substantially lower black contents, with the objects, inter alia, of improving flexural and other physical properties and substantially increasing the ratio Rp/Ri. However, attainment of these goals has in large part been precluded by the extremely high room temperature resistivities exhibited by polymers with low black loadings. In Cabot Corporation's Pigment Black Technical Report S-8, entitled "Carbon Blacks for Conductive Plastics" percent carbon-resistivity curves for various polymers containing "Vulcan XC-72", an oil furnace black, show resistivities of 100,000 ohm-cm or more, asymptotically increasing at black loadings of about 15%. Others have reported similarly high resistivities with low black loads. Recently resistivities sufficiently low for freeze protection applications have been achieved with low black loadings by resort to the special deposition techniques, such as solvent coating, disclosed in commonly assigned copending U.S. patent application Ser. No. 88,841, filed Nov. 12, 1970 by Robert Smith-Johannsen (now abandoned). Self-limiting compositions have been extruded heretofore, e.g., U.S. Pat. No. 3,435,401 to Epstein, but when low black loading has been attempted the extrudates have exhibited room temperature resistivities of 10.sup.7 ohm-cm or higher, essentially those of the polymer matrices themselves. Indeed, the patentees of G.B. Pat. No. 1,201,166 urge the avoidance of hot melt techniques where significant conductivities are desired with less than about 20% black.